The invention relates to electrical converters, especially for use in power supplies in electrically propelled vehicles with external power pick-up, e.g., in electrical locomotives, streetcars, trolley buses, and subways.
Electrically propelled vehicles may require one or several auxiliary power supplies, e.g., for lighting, air conditioning, and communications. Typically, such power supplies run off a main DC line supplying power to the traction drive system of the vehicle. Electrical connection to the main power line is through a carbon brush to an overhead wire or a third rail; in the case of trolley buses, two carbon brushes to overhead power lines are used. As a result, due to the intermittent nature of connection through moving contacts, DC input power is subject to large voltage transients.
Auxiliary power requirements on the electric vehicle may be AC or/and DC, and often at several different voltage levels. For example, a vehicle may have a 37-volt DC low-voltage bus, a 370-volt DC high-voltage bus, and a 230-volt 3-phase AC bus. These line characteristics may be derived from three separate supply circuits or from a single hybrid circuit. In either case, power is extracted from the main power line at 600 volts DC, for example.
Known power supply circuit configurations of interest here have evolved into topologies which may be referred to as GTO-AC and GTO-DC type, for AC and DC output power, respectively. In a GTO-AC power supply, a GTO (gate turn on) switching network drives the primary side of a low frequency transformer, typically at 60 Hz. The secondaries of the transformer provide the desired AC power to a bus in the vehicle. In this design, the transformer turns ratio must be chosen to give the desired AC output voltage for the 600-volt DC nominal input. Any long-term transient will show up in the output of the secondary winding, so that the output voltage will be regulated only if the input voltage is regulated. Moreover, even at constant input voltage, the output voltage will vary somewhat with load current.
A GTO-DC power supply is similar, with a GTO switching network on the primary side of a transformer inducing a low-frequency AC output voltage at the output of the secondary winding(s). This voltage is then fed into a rectifier network to provide a DC source. Again, the output voltage varies with input voltage and with load current.
These topologies have gained acceptance because of their ruggedness and proven reliability. Transient voltages on the 600-volt line input can be in the range of 3000 to 5000 volts, and any equipment such as GTO's directly connected to this line must be able to survive the transients. Of course, a filter may be interposed for attenuating the large transients. Since the transients have a significant amount of energy, the filter must have large capacity. As a result, for a 15 kW power supply, for example, such a filter will weigh several hundred pounds. If a smaller filter is to be used, GTO's are required with correspondingly higher voltage ratings.
Although these power supplies have proved to be reliable in the field, they have significant drawbacks. One significant drawback lies in their size and weight. Since they operate at low frequency, all their energy conversion components, including the input filter chokes, input filter capacitors, transformer, output filter chokes and output filter capacitors are very large and bulky, typically having a weight upward of 2000 pounds and a volume on the order of 50 cubic feet.
Power supplies in which the supply frequency is higher have reduced weight and size, with smaller and lighter filter capacitors and switching transformers. Such power supplies require transistors which can switch at high frequencies. While GTO's can operate only up to 1 kHz, high-speed transistors such as insulated gate bipolar transistors (IGBT) and metal oxide semiconductor field effect transistors (MOSFET) can switch at frequencies up to at least 20 kHz; see, e.g., U.S. Pat. No. 4,860,184, issued Aug. 22, 1989 to W. A. Tabisz et al.
High-speed transistors are available only in voltage ratings up to 1200 volts, so that use of IGBT's or MOSFET's in vehicular applications again would seem to require a large input filter to attenuate the transient voltages on the traction power line. Such combinations of an input filter with a high frequency switching converter are not price competitive with low frequency GTO-type converters.